With the discovery of the availability of optical fibers which exhibit extremely low attenuation to long wavelength light, e.g., the range of approximately 1.0 microns to 1.7 microns, there has developed a need for fast photodetectors for use at such wavelengths.
In particular, it was previously known that imaging, sensitive at these wavelengths, could be provided utilizing silicide Schottky-barrier detectors fabricated on silicon wafers. See, for example, the article by E. S. Kohn, entitled "A Charge-Coupled Infrared Imaging Array with Schottky-Barrier Detectors" appearing in the IEEE Journal of Solid-State Circuits, Vol. SC-11, No. 1, February 1976, pp. 139-146. These detectors comprise a palladium silicide layer on a p-type silicon crystal to form a Schottky-barrier diode. When such a diode is biased in reverse and infrared light is made incident on the semiconductor for passage therethrough and absorption in the metal, it can act as a photodetector. Carriers formed in the silicide by the absorption of the infrared are photoexcited over the barrier from the silicide into the silicon where they become majority carriers and form a photocurrent.
It was recognized that the quantum efficiency of this process is low but this shortcoming was compensated for by the relative independence of the sensitivity to such parameters as semiconductor doping and minority carrier lifetime.
Moreover, the response extends to photon energies as low as the barrier height, a value that can be considerably smaller than the bandgap in silicon, permitting operation at wavelengths much longer than with the ordinary silicon photodetector and, accordingly, in a range of interest.
Our interest in photodetection is primarily for use in optical transmission systems where the optical pulses transmitted need to be detected and converted to electrical signals either for use in repeaters for the regeneration of the optical pulses for further transmission or for utilization at the signal's final destination. In such applications it is usually necessary to amplify the electrical signal after detection.
Moreover because of the low level of the detection signals in such silicide Schottky-barrier detectors, it is important for transmission applications that the amplification be done without adding significant noise. In accordance with a feature of our invention, amplification is achieved by integrating a photodiode stage monolithically with an amplifier stage to achieve a high signal-to-noise ratio at the output of the integrated circuit. In our preferred embodiment the diode stage and the amplifier stage are merged in a device to be called a permeable base phototransistor.
In a typical embodiment of the invention, the photodiode comprises a silicide layer on a silicon crystal substrate and the amplifier is a silicon transistor which utilizes, as the control element to which the input is applied, an electrode which is effectively an electrical extension of the silicide layer of the photodetector. Moreover the transistor also utilizes as the controlled conduction channel, which controls the output branch, a silicon body which is also part of the silicon crystal of the photodiode.
In a preferred embodiment the amplifier is a permeable grid silicon transistor in which the permeable grid is a silicide layer which is epitaxial both with underlying and overlying silicon layers, between which the carrier flow is controlled by the silicide grid. This grid is adapted to absorb the optical signal to be detected and to develop from it a photovoltaic signal which is amplified. In this structure, the photodiode stage is effectively merged with the amplifier stage. The dopings in the silicon layers are designed to encourage avalanche multiplication of the photocarriers generated.
In one aspect the invention is a permeable base phototransistor.
Other embodiments are described utilizing MOS and Schottky-barrier field-effect transistors.